The present invention is directed to control devices for bicycles and, more particularly, to an automatic shifting device for shifting a bicycle transmission.
There are two major types of bicycle transmissions currently marketed. One type of bicycle transmission uses a derailleur to shift a chain from one sprocket to another to change the drive ratio, and the other type uses a planetary gear mechanism disposed inside a rotating hub such as a wheel hub to change the drive ratio. In either case, an operating member such as a lever, push rod or rotatable ring is moved to operate the transmission, and the operating member is connected to a transmission actuating member such as a control wire or link that is pulled or released by the rider to cause the desired movement of the operating member. To facilitate the operation of the transmission in a pull and release manner, a return spring commonly is used to bias the operating member to an initial state. The transmission then is operated by moving the operating member against the progressively increasing force of the return spring and the natural resistance by the transmission to changing gears.
When the transmission actuating member is a control wire, the control wire ordinarily is connected to a shift control device mounted to the bicycle handlebar. The shift control device usually includes a takeup member that winds and unwinds the control wire to perform the pulling and releasing function, and a rotating ring or lever to rotate the takeup member. After a winding operation has been performed to select a desired transmission gear, the takeup member must be maintained in the position corresponding to that gear. Known structures for positioning the takeup member include those merely involving the use of frictional resistance; those in which indentations are formed in the speed-step positions on the side of the fixed elements for engaging protrusions formed on the side of the control members (as described in Japanese Laid-Open Patent Application 3-176290); and those involving the provision of one-way ratchet mechanisms (as described in PCT/JP92/00138).
In shift control devices where positioning is accomplished using frictional resistance or engagement between indentations and protrusions, considerable engagement force is needed to securely maintain the takeup member in the desired position. When the resistance of the return spring is added to the resistance created by such position maintaining structures, the force that must be applied to operate the shift control device can become quite large.
Motor driven bicycle transmissions are becoming more common. Such motor driven bicycle transmissions relieve the rider of the task of manually pulling and releasing the transmission actuating member. Of course, the motor used to cause the pulling and releasing of the transmission actuating member must be designed to withstand the forces noted above. This sometimes makes it necessary to use a relatively large motor which increases the cost, size and weight of the transmission operating mechanism. The larger motor also creates significantly more current drain on the battery used to operate the motor, thus resulting in short battery life. Consequently, motor driven transmissions usually are not used when the size, cost or weight of the bicycle is an important factor, such as in racing environments.